Purification of human vitamin K-dependent protein S and its limited proteolysis by thrombin

Abstract

Vitamin K-dependent protein S exists in two forms in human plasma, namely as the free protein and in complex with C4b-binding protein. Now reported is a simple purification procedure for human protein S that includes barium citrate adsorption, DEAE-Sephacel chromatography and chromatography on Blue Sepharose. The yield was approx. 30% relative to the concentration of free protein S in plasma, which was found to be approx. 10 mg/l. Purified protein S migrated as a single-chain band on sodium dodecyl sulphate/polyacrylamide-gel electrophoresis under non-reducing conditions and as a doublet of M(r) approx. 85 000. and 75 000 on reduction. A third band of M(r) 16 000 was observed after electrophoresis of 125I-labelled protein S and radioautography of reduced samples. This band appears to be disulphide-linked to the 75 000-M(r) chain before reduction. Thrombin converted the 85 000-M(r) chain of protein S into a 75 000-M(r) chain and an 8000-M(r) fragment, the latter again being detectable only by radioautography of reduced samples. The 16 000-M(r) fragment was not observed, suggesting its degradation by thrombin. Under non-reducing conditions, no change in apparent molecular weight of thrombin-treated protein S was observed, indicating disulphide linkage of the fragments. Thrombin also affected the mobility of protein S on agarose-gel electrophoresis in the presence of Ca2+, suggesting a decreased affinity to Ca2+ of the cleaved form of protein S as compared with the undegraded molecule. After activation of the complement system in human serum, protein S was found to be a constituent part of the complex formed by C4b-binding protein and component C4b.